Oh I hate that. Check out Autozone or Advance, they have TONS of stop leak stuff to plug up your radiator.

Unfortunatley, it also plugs up your heater core and water pump.

The BEST solution to stop it from leaking is to replace it.

Do you know exactly where the leak is coming from?

Check out a few different shops, alot of places will replace the side tanks if its leaking from there.

It shouldnt be too terribly expensive to replace the whole radiator